1. Field of the Invention
The invention relates to a method for fabricating a ferroelectric integrated circuit that reduces or eliminates the degradation of electronic properties resulting from hydrogen exposure.
2. Statement of the Problem
Ferroelectric compounds possess favorable characteristics for use in nonvolatile integrated circuit memories. See Miller, U.S. Pat. No. 5,046,043. A ferroelectric device, such as a capacitor, is useful as a nonvolatile memory when it possesses desired electronic characteristics, such as high residual polarization, good coercive field, high fatigue resistance, and low leakage current. Lead-containing ABO.sub.3 type ferroelectric oxides such as PZT (lead titanate zirconate) and PLZT (lanthanum lead titanate zirconate) have been studied for practical use in integrated circuits. Layered superlattice material oxides have also been studied for use in integrated circuits. See Watanabe, U.S. Pat. No. 5,434,102. Layered superlattice compounds exhibit characteristics in ferroelectric memories that are orders of magnitude superior to those of PZT and PLZT compounds. While prototypes of ferroelectric memories have been made successfully with the layered superlattice compounds, there is as yet no manufacturing process for making memories using either the ABO.sub.3 type oxides or the layered superlattice material compounds with the desired electronic characteristics economically and in commercial quantities. One reason, among others, for the lack of economical commercial processes for the fabrication of high quality ferroelectric integrated circuits is that the oxide compounds are susceptible to reduction by hydrogen during hydrogen annealing. Hydrogen annealing is a common step during CMOS integrated circuit memory fabrication and results in degradation of some important ferroelectric properties.
A typical ferroelectric memory device in an integrated circuit contains a semiconductor substrate and a metal-oxide semiconductor field-effect transistor (MOSFET) in electrical contact with a ferroelectric device, usually a ferroelectric capacitor. A ferroelectric capacitor typically contains a ferroelectric thin film located between a first or bottom electrode and a second or top electrode, the electrodes typically containing platinum. During manufacture of the circuit, the MOSFET is subjected to conditions causing defects in the silicon substrate. For example, the manufacturing process usually includes high energy steps, such as ion-mill etching and plasma etching. Defects also arise during heat treatment for crystallization of the ferroelectric thin film at relatively high temperatures, often in the range 500.degree.-900.degree. C. As a result, numerous defects are generated in the single crystal structure of the semiconductor silicon substrate, leading to deterioration in the electronic characteristics of the MOSFET.
To restore the silicon properties of the MOSFET/CMOS, the manufacturing process typically includes a hydrogen annealing step, in which defects such as dangling bonds are eliminated by utilizing the reducing property of hydrogen. Various techniques have been developed to effect the hydrogen annealing, such as H.sub.2 -gas heat treatment in ambient conditions. Conventionally, hydrogen treatments are conducted between 350.degree. and 550.degree. C., typically around 400.degree. C. for a time period of about 30 minutes. In addition, there are several other integrated circuit fabrication processes that expose the integrated circuit to hydrogen, often at elevated temperatures, such as CVD processes for depositing metals, growth of silicon dioxide from silane or TEOS sources, and etching processes using hydrogen. During processes that involve hydrogen, the hydrogen diffuses through the top electrode and the side of the capacitor to the ferroelectric thin film and reduces the oxides contained in the ferroelectric material. The absorbed hydrogen also metallizes the surface of the ferroelectric thin film by reducing metal oxides. As a result of these effects, the electronic properties of the capacitor are degraded. This problem is acute in ferroelectric memories containing layered superlattice compounds because these oxide compounds are particularly complex and prone to degradation by hydrogen-reduction.
3. Solution to the Problem
The invention solves the above problem by providing a method for fabricating ferroelectric elements in integrated circuits that reduces the detrimental effects of the hydrogen and preserves the favorable electronic properties of the ferroelectric element.
One aspect of the invention is performing an oxygen-recovery anneal to reoxidize chemical compounds in the ferroelectric element that were reduced during manufacturing steps causing hydrogenating and reducing conditions. The oxygen-recovery anneal is typically performed in the temperature range from 300.degree. C. to 1000.degree. C. for a time period from 20 minutes to 2 hours.
Another aspect of the invention is that the oxygen-recovery anneal is ambient tolerant, that is, it can be performed at atmospheric pressure with oxygen gas mixtures containing common ambient gases, such as nitrogen.
Another aspect of the invention is formation of a hydrogen barrier layer directly over at least a portion of the ferroelectric element.
In a preferred method, a nitride of titanium or silicon is formed to cover the protected portion of the ferroelectric element and serve as a hydrogen barrier.
Another aspect of the invention is formation of a ferroelectric thin film comprising a layered superlattice compound.
Another aspect of the invention is forming a ferroelectric element having layered superlattice compounds containing the chemical elements bismuth, strontium, niobium and tantalum in which the relative amounts of the chemical elements are selected to minimize the degradation of electronic properties by hydrogen.
Another aspect of the invention is formation of a ferroelectric thin film in which the layered superlattice compound comprises strontium bismuth tantalum niobate.
Another aspect of the invention is formation of a ferroelectric thin film in which the ferroelectric thin film comprising strontium bismuth tantalum niobate contains relative amounts of niobium and tantalum selected to inhibit hydrogen degradation of the ferroelectric material.
Another aspect of the invention is formation of a ferroelectric thin film in which at least one of said metals is present in an excess amount up to forty percent greater than the amount corresponding to a balanced stoichiometric formula.
Another aspect of the invention is formation of a ferroelectric thin film in which the ferroelectric thin film comprising strontium bismuth tantalum niobate contains an excess amount of at least one of the metals from the group comprising bismuth and niobium to inhibit hydrogen degradation of the ferroelectric material.
Another aspect of the invention is performing a hydrogen heat treatment in an atmosphere including hydrogen at a temperature of 350.degree. C. or less, for a time period not greater than 30 minutes, wherein the mole fraction of hydrogen in said hydrogen atmosphere is from 0.01 to 50 percent.
A further aspect of the invention is the formation of a ferroelectric capacitor with a top electrode, a ferroelectric thin film and a bottom electrode.
Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.